This invention relates to refrigerant compressors driven by an electrical motor capable of operating at a low speed and a high speed and to the method and means for controlling the refrigerant compressor utilizing a two-speed electric motor having four separate windings, a two-pole auxiliary and main, and a four-pole auxiliary and main winding, all wound on the same core.
U.S. Pat. Nos. 1,609,310--Rodgers and 1,712,065--Baker teach two speed motors wherein windings are arranged for two separate speeds of the motor. Refrigerant compressor utilizing two-speed motors have been employed in the past. One such arrangement is shown in U.S. Pat. No. 3,584,980--Cawley et al, which discloses that a four-pole, two-winding electric motor can be utilized to provide full capacity during two-pole operation and one half capacity during four-pole operation and means for providing adequate lubrication of the crankshaft bearing surfaces during both high speed and low speed operation. U.S. Pat. No. 3,943,912 is directed to the method of controlling a two-speed motor of the type disclosed in U.S. Pat. No. 3,584,980 wherein the operation of the two-speed motor compressor system can be controlled such that the overall coefficient of performance (efficiency) of the system at the lower speed, i.e., during part load conditions, is equal to or greater than the coefficient of performance (efficiency) of the system at the higher speed during peak load conditions. A first torque load is imposed on the two-speed motor during peak load operation and under a given set of evporating and condensing conditions, and then a second torque load is imposed on the motor during part load operation and under the same set of evaporating and condensing conditions such that the ratio of the second torque load to the first torque load is equal to or less than the ratio of the motor efficiency at part load operation to the motor efficiency at peak load operation.
Other prior art teachings of a refrigerant compressor utilizing two-speed motors are shown in U.S. Pat. No. 3,978,382--Pfarrer and 4,041,542--Pfarrer, wherein temperature sensors are located adjacent each of the windings for detecting any change in temperature of the windings. A current transformer is used in combination with the temperature sensor and in the event the temperature or current conduction of any of the windings exceeds a predetermined value, a control device operates a switch in a pilot circuit that turns off power to the motor.
Another prior art teaching of a refrigerant compressor utilizing a two-speed motor is shown in U.S. Pat. No. 4,064,420--Yuda et al, which provides a control system for a pole-changing, motor-driven compressor that stops the motor in case of the pole changing thereof and starts it only after the operating conditions of the compressor change to permit the restarting of the motor, whereby the reliable and dependable pole-changing operation can be ensured and the motor may be prevented from being damaged.
An object of the present invention is to provide a system and method of controlling the operation of a hermetic two-speed, electric motor-driven refrigerant compressor so as to attain an overall motor efficiency which is substantially equal at either low or high speed operation, while providing higher overall compressor efficiency at the lower speed.
Another object of the invention is the provision of a control system wherein low speed operation of the compressor motor is controlled through a first stage switch means of the indoor thermostat.
Another object is to provide for mandatory starting of the motor in low speed and to continue low-speed operation for a predetermined period of time.
Another object is to provide for instantaneous speed switching between speeds without interrupting compressor operation.
Another object is to provide separate, internal, line break, motor protectors for each speed winding that can be optimized for each winding without affecting the other.
This and other objects and advantages of the present invention will become apparent from the following description of the preferred embodiment of the invention described in connection with the accompanying drawing.